Modern vehicles generally have a tank ventilation system which collects fuel vapors from the tank system in an activated carbon filter when the vehicle is parked. While the vehicle is operating, the activated carbon filter is regenerated, a valve of a line from the activated carbon filter to the suction pipe being opened by a defined amount so that air from the activated carbon filter enters the tank ventilation system and feeds the stored hydrocarbons to the intake air of the engine. This mixture that is additionally fed for combustion leads to a change in the overall composition of the mixture and the filling of the engine. This change can be countered by suitable control mechanisms or a suitable precontrol.
A known method measures the concentration of hydrocarbons in the tank ventilation gas flow and corrects the quantity of fuel introduced via the injection valves by the amount of the fuel quantity additionally introduced through the tank ventilation. This procedure is referred to below as injection correction by the tank ventilation. An important point in injection correction by tank ventilation is always the question of the point in time at which the injection correction should commence. This is explained briefly below:
If the valve is opened after a lengthy regeneration interruption or for the first time after the engine is started, the concentration of fuel vapors is not known, and a correction cannot be carried out. As soon as a measurable mixture deviation, i.e. a deviation between lambda actual value and lambda set value exists, estimation of the concentration can commence. However, at the time of the measurable mixture deviation at the lambda probe, a certain quantity of regenerating gas is already in the suction pipe, cylinders and exhaust tract.
The opening point of the valve, i.e. the point in time at which the valve is actuated for ventilation, and consequently a measurable deviation of the mixture arises, is subject to tolerances. The tolerances are so large that a reliable precontrol of the fuel correction that is based on a predetermined opening time of the valve is not possible. Also, the opening behavior of the valve can be non-linear; for example it is possible that the valves will open suddenly only upward of a certain signal strength in the control. The supply of the regenerating gas can therefore be dosed only poorly.
In the known methods, a significant enrichment of the mixture after opening of the tank ventilation valve was previously unavoidable, particularly in the idling range and at lower load levels.
In order to avoid sudden changing of the composition of the mixture, it was proposed that the enrichment be minimized by a slow opening of the valve. However, this prolongs the opening phase of the valve and enrichment of the mixture cannot fully be avoided due to the sudden behavior of the valve flap.
The technical object of the invention is to provide a method for tank ventilation which using simple means determines very reliably a suitable time and thus also a suitable quantity of fuel, depending on the operating status, for the commencement of an injection correction by the tank ventilation.